1. Field of the Invention
The present invention relates to a visual acuity testing apparatus which projects an image of a chart for visual acuity testing onto a screen and tests visual acuity of an examinee's eye.
2. Description of Related Art
Conventionally, there is known a projection-type visual acuity testing apparatus which projects an image of a chart for visual acuity testing onto a screen placed in a position at a predetermined distance (a test distance) from an examinee. In this apparatus, a projection lens is moved in accordance with positions of the screen, the apparatus, and the examinee. In one type of the apparatus, an installation distance (a distance from the screen to the apparatus, also called as a projection distance) and the test distance are equal, and an adjustment is performed so that an image of the chart having a predetermined size corresponding to these distances is projected onto the screen (see Japanese Patent Application Unexamined Publication No. 2003-310552 (hereinafter referred to as patent document 1)). The scheme of patent document 1 is called a fixed power scheme. Meanwhile, in another type of the apparatus, the size of an image of the chart projected onto a screen can be zoomed in or out by moving a plurality of projection lenses (see U.S. Pat. No. 3,947,098 corresponding to Japanese Patent Application Unexamined Publication No. Sho50-37293 (hereinafter referred to as patent document 2)). The scheme of patent document 2 is called a variable power scheme.
In the apparatus using the fixed power scheme, the size of the chart image projected onto the screen is uniquely determined in accordance with the installation distance and the test distance. Additionally, projection light intensity of the chart image projected onto the screen is adjusted in accordance with the installation distance and the test distance. This adjustment is performed so that the examinee perceives that brightness of the chart image is substantially constant at a reference value.
Meanwhile, in the apparatus using the variable power scheme, the installation distance and the test distance are not necessarily required to be equal. In other words, the installation distance and the test distance can be set independently. The size of the chart image projected onto the screen is determined in accordance with the set installation distance and test distance. Projection light intensity of the chart images is not adjusted in any conventional apparatuses using the variable power scheme.
As an allowable range of the international standard for brightness of chart images is relatively wide, no problems are particularly indicated regarding the foregoing apparatus (which is incapable of adjusting the projection light intensity of the chart images). However, when the test distance and the installation distance vary, the brightness of the chart image varies. This means that tests are performed under different conditions, thereby possibly influencing test results.